1. Field of the Invention
This invention relates to a quadrature amplitude modulation (QAM)-based communication system and, more particularly, to interference suppression in a digital QAM demodulator.
2. Description of the Background Art
Quadrature amplitude modulation (QAM) is a particularly advantageous technique for transmitting digital data because of its efficient utilization of bandwidth. As an example, high definition television (HDTV) signals are oftentimes broadcast as compressed digital data using QAM.
In essence, QAM transmits digital data as a sequence of two-dimensional complex symbols which may be expressed in terms of level and phase, or equivalently, in terms of in-phase and quadrature components. Each symbol, based upon the data represented by the symbol, takes on a specific pre-defined value from a set of values. The set of all values, when graphically plotted in two-dimensions, forms a so-called constellation. The size and shape of the constellation depends upon the number of discrete values in the set and their spatial location in the constellation. The constellation might contain, for example, 16 or 64 values, hence called 16 QAM or 64 QAM, respectively.
To broadcast QAM, the in-phase and quadrature digital components independently modulate in-phase and quadrature carrier signals, respectively, and the modulated carriers are propagated over the given channel or medium (e.g., cable or "over-the-air" broadcast).
To detect an incoming QAM signal, a QAM receiver demodulates the in-phase and quadrature incoming sampled signals using carrier signals derived from a carrier recovery circuitry, and the demodulated outputs are filtered, with the filtered signals serving as inputs to an appropriate decoder which typically utilizes slicer circuitry to produce detected symbols.
The incoming signals to the QAM receiver are provided, for example, over broadcast channels or cable systems. One deleterious type of interference which affects the desired incoming signal is a discrete, in-band radio-frequency (RF) tone. A low power RF tone is particularly troublesome for high-order constellations because of their compactness. Previously known techniques for interference cancellation, typically implemented at the front end of the receiver, are not particularly effective because these techniques rely upon substantial power in the interfering tone. The RF tone interference produces a significant error rate by causing perturbations of the constellation points. Such interference is not atypical and may arise on a cable system from sources such as crosstalk from co-channel NTSC broadcasts or beats from NTSC carriers on the same cable system.
Techniques which address interference suppression of co-channel NTSC interference signals into QAM signals are known for broadcast applications. Representative of these techniques are the disclosures of related U.S. Pat. Nos. 5,282,023; 5,325,188; 5,325,204; and 5,400,084. The underlying technique of these references utilizes a bank of narrow band IIR filters to isolate the interfering signal and subtract it from the desired signal. This technique is accomplished at the front end of the QAM demodulator, that is, ahead of any other processing. This technique is especially suitable for high power interference, but it is less effective at detecting and removing low power interference which may still be a problem for QAM signals having a relatively large number of symbol states, that is, high order constellations. Moreover, this technique requires complex circuitry for its implementation.
Other art, as set forth in U.S. Pat. Nos. 5,087,975 and 5,162,900, for canceling NTSC co-channel interference in a vestigial sideband pulse amplitude modulated system relies on special precoding at the transmitter and a fixed filter in the demodulator. Such a technique is not easily generalized to QAM and not applicable for solutions implemented only in the receiver.
Finally, other techniques for interference cancellation have been discussed in the literature; a survey of these techniques is covered in the article entitled "Adaptive Noise Cancelling: Principles and Applications," by Widrow et al, Proc. IEEE, Vol. 63, No. 12, pages 1692-1716, December 1975. These techniques generally rely on the availability of a correlated reference signal for the interference; such a reference signal is derived from a second receiver or generated from known properties of the signal, neither of which is known or available in a typical QAM application.
Thus, the prior art is devoid of teachings or suggestions for suppressing low-level discrete RF tone interference in a QAM system which is the focus of the present invention.